The seller, believed to be Waldeck, says the "one-of-a-kind 3.56 carat Ceylon sapphire gem and diamond ring has been certifiably valued at $19,150."
The seller adds that the ring is in "perfect condition with no marks or scratches" and describes it as having been custom-made from Wishart Jewellers.
The former couple started dating soon after she appeared on the inaugural season of The Bachelor in 2013.
In 2015, Ali gushed about her then-new man.
"Tim absolutely led me to Dave and he is the most perfect man in the world," she told news.com.au prior to her engagement.
She added: "The Bachelor was my hurdle and my challenge and something I learnt from and made me more of who I am today and Dave is the one for me."
Dave proposed in June 2015, after he and Ali enjoyed a romantic trip to Europe together.